A heating unit of this general type has been disclosed in my prior U.S. Pat. No. 4,140,482. Such a unit has a burner housing receivable in a wall of the furnace to be heated thereby, the burner housing comprising three coaxial tubes; the innermost and the intermediate tubes respectively supply a fuel gas and air to a combustion space while exhaust gases from that space flow in the opposite direction through the outermost tube. In that prior system, the incoming combustion air is preheated mainly in a collateral heat exchanger which it traverses in counterflow to the exhaust gases.
Such a heating unit operates satisfactorily with highly calorific fuel gases but is less effective with lean gases whose calorific value lies in a range of about 3 to 7 MJ/m.sup.3.